Electron beam apparatus comprising a cathode to be heated by an energy beam

ABSTRACT

An electron source comprising a cathode in the form of a metal wire placed under tension and heated by an energy beam. The energy beam laterally impinges upon the metal wire which has a lateral emitting surface. The metal wire can be displaced along the location where the energy beam impinges without a slackening of the wire. This movement can be automatically controlled in response to the temperature of the metal wire.

United States Patent [191 Le Poole ELECTRON BEAM APPARATUS COMPRISING A CATHODE TO BE HEATED BY AN ENERGY BEAM [75] lnventor: Jan Bart Le Poole, Delft,

Netherlands [73] Assignee: U.S. Philips Corporation, New York,

221 Filed: Dec. 1, 1971 21 Appl. No.: 203,708

[30] Foreign Application Priority Data Dec. 23, 1970 Netherlands 7018701 [52] US. Cl 250/495 A, 313/237, 313/278, k 313/337, 313/347 [51] Int. Cl H0lj 37/26, (10111 23/00 [58] Field of Search 250/495 R, 49.5 A;

1 1 3,745,342 1 July 10,1973

[56] References Cited UNITED STATES PATENTS 3,506,871 4/1970 Hunt 313/278 3,389,252 6/1968 Le Poole .1 250/495 FOREIGN PATENTS OR APPLICATIONS 1,028,244 4/1958 Germany 250/495 Primary Examiner-William F. Lindquist Attorney-Frank R. Trifari [57] ABSTRACT An electron source comprising a cathode in the form of a metal wire placed under tension and heated by an energy beam. The energy beam laterally impinges upon the metal wire which has a lateral emitting surface. The metal wire can be displaced along the location where the energy beam impinges without a slackening of the wire. This movement can be automatically controlled in response to the temperature of the metal wire.

14 Claims, 2 Drawing; Figures Patented July 10, 1973 3,745,342

DET/ (MOTOR omve CONTROL MOTOR INVIZIV'I'U}; JAN 8. LE POOLE ELECTRON BEAM APPARATUS COMPRISING A CATHODE TO BE HEATED BY AN ENERGY BEAM The invention relates to an electron beam apparatus comprising an electron source for generating an electron beam, said electron source being provided with a metal wire which can be displaced in the longitudinal direction, and comprising means for directing an energy beam onto a portion of the metal wire.

An electron beam apparatus of this kind is known from German Patent Specification 1,028,244. In an electron beam apparatus described therein an end face of a metal wire is heated by an incident ion beam. The shifting of the emissive surface due to evaporation and sputtering of cathode material at the area of the emissive surface is eliminated by displacing the metal wire in the direction of the emissive end face. The described electron beam apparatus has for its object to reduce the energy to be supplied to the cathode in comparison with a filament cathode heated by an electric current, and to improve the potential distribution near the cathode for electron-optical purposes. The latter is achieved in that the ion beam exerts a neutralizing effect on the space charge in front of the emissive cathode surface. The end of the metal wire is heated to a temperature which is normal for filament cathodes. A significantly higher temperature cannot be realized in this apparatus because the emissive surface then exhibits instabilities.

The invention has for its object to provide an electron beam apparatus'whose electron source has an emissive surface which is co-determined by the dimensions of the metal wire, the current density of the electron beam at the cathode surface having a high value with respect to known cathodes. To this end, an electron beam apparatus of the kind set forth according to the invention is characterized in that the energy beam is directed onto a side of a metal wire whichis tensioned under a tensile force acting in the longitudinal direction, the emissive region extending over a side of the metal wire.

In an electron beam apparatus according to the invention the metal wire can be heated to a comparatively high temperature without the risk of instabiliities. By choosing the metal wire'to be thin one dimension of the emissive surface is determined by the thickness dimension of the metal wire. The dimension of the emissive surface in the longitudinal direction of the metal wire can be adjusted to a desired value by choosing the width of the energy beam where it impinges upon the wire or by mechanicalmeans. An elongated emissive surface having small dimensions in one direction and a high current density for the electron beam to be emit ted can thus be readily realized.

In a preferred embodiment according to the invention the metal wire is moved in the longitudinal direction during operation, thus prolonging the service life of the electron source.

In order that the invention may be readily carried into effect, one embodiment thereof will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIG. 1 is a diagrammatic representation of a scanning electron microscope according to the invention,

FIG. 2 is a more detailed perspective drawing of the electron source of the scanning electron microscope shown in FIG. 1.

Situated in the scanning electron microscope shown in FIG. 1 are an electron source 1 comprising a cathode 2, a first anode 3 and a second anode 4. Proceeding in the direction of the electron beam 5 emitted by the cathode 2 we find, as in known scanning electron microscopes, a device 6 directing the electron beam 5, a condenser lens 7, a condenser diaphragm 8, an imaging lens 9, and a deflection system 10 comprising deflection coils 11 and deflection coils 12 for scanning the electron beam across an object 13 in the image direction and in the line direction. A diaphragm 14 is situ-- ated between the deflection-coils and the object or specimen 13. A detector 15 is arranged with respect to the object such that it at least partly intercepts signals emerging from the object either inthe form of particles or in the form of electromagnetic radiation. A cathode ray tube 16, whose deflection coils l7 and 18 operate in synchronism with the deflection coils 11 and 12, serves for imaging the signals received by the detector. Situated between the detector 15 and the cathode ray tube 16 is an amplifier 19 for amplifying the detector signal by which the cathode ray tube is controlled. The cathode 2 is formed by a metal wire which is assumed to extend perpendicular to the plane of the drawing in FIG. 1. This metal wire is laterally :irradiated by an electron beam 20, and is thus heated to a temperature of, for example, 3,400C for a tungsten wire for which a normal emissive temperature of approximately 2,800C is maintained. As the metal wire is tensioned under a tensile force, the wire remains straight even at this elevated temperature. A temperature increase from 2,800'C to 3,400C in a tungsten wire causes an increase of the current density of the electron beam to be emitted from approximately 2 to 5 A per cm to approximately 50 to 200 A per cm. In the preferred embodiment the energy beam for heating the metal wire consists of an electron beam 20 which is emitted by a needle cathode 21, is adjusted to a desired current intensity of, for example, 25 A at an incident energy of 10 kV by a control grid 22, and is focussed to acrosssection of, for example, 5 to 25 microns on one side of the metal wire 2 by a lens 23. The lens 23can be advantageously formed by a minilens as described in US. Pat. No. 3,394,254. The energy beam may also be formed by a laser beam,-an ion beam or another energy beam of a corpuscular or electromagnetic nature. In

, the preferred embodiment the electron beam and the energy beam enclose an angle of For this angle any other arbitrary .value may be chosen, coinciding but op positely directed beams not being precluded in this respect. i i i FIG. 2 is a schematic view of the electron source 1 of the preferred embodiment shown in FIG. 1. The metal wire, having a thickness of, for example 5 to 50 microns, extends under a tensile force acting in the longitudinal direction and keeping the wire straight under all circumstances, between a reel 25 with a guide 26 and a reel 27 with a guide 28. About the metal wire are situated two supports 29 and 30, each of which is provided with a Vee groove 31 and 32, respectively, ensuing that the metal wire 2 is accurately guided in the longitudinal direction. 'The supports preferably make proper thermal contact with the metal wire, for which purpose, for example, the Vee grooves tightly envelop the wire, the supports consisting of a metal having favourable thermal conducting properties such as, for example, silver. Situated between the supports and around the wire is an anode 3 which in this case has the shape of a hollow cylinder having an inner diameter of, for example, 0.5 to 2 mm and a wall thickness 35 of, for example, 0.2 to 0.5 mm. This cylinder is provided with an aperture 33 for discharging the electron beam emitted by the metal wire, and with an aperture 34 for injecting the energy beam for heating the metal wire. The aperture 34 will preferably be circular, while the aperture 33 can be adapted to the requirements for the dimensions of the electron source in the longitudinal direction, so it may be, for example, 50 X 500 microns. When the potential of the metal wire 2 is zero during operation, the anode will have a potential of, for example, +100 to +500 volts.

The embodiment of the electron source shown in FIG. 2 merely serves for the purpose of illustration. For

the invention it is irrelevant as to how the metal wireis moved in operation, if moved at all. The movement can also be realized by displacing a metal wire, tensioned under resilience in a carrier having a length of, for example, 5 to cm, together with the carrier along guides, for example, via an externally operable spindle. For the metal wire, whose cross-section may have any desired shape, other metals having a high melting temperature mayalternatively be used, for example, titanium. At the high temperature to which the metal wire is brought the metal evaporates compartively quickly. Consequently, materials having a comparatively low evaporation rate at a temperature just below the melting point are to be preferred. The heated area of the wire, viewed in the longitudinal direction, is localized by the supports. In some cases it will be advantageous to construct the supports such that the anode tube protrudes partly therein so that the space between the anode tube and the supports is closed. The supports and the anode tube may also form one mechanical assembly. The displacement of the metal wire serves for increasing the service life of the electron source, which is determined mainly by evaporation of the metal wire. By adapting the rate of displacement to the incident energy, it can be ensured that always one portion of the metal wire is at the desired temperature, and that the metal wire is not excessively evaporated anywhere. The metal wire is cooled by conduction, forced by the metal blocks, by radiation, which is intercepted mainly by the anode tube, and by the emission of electrons. It was found that at these current densities the heat discharge caused by the emission is higher (for example, by a-factor 10) than that caused by radiation. Consequently, a more homogeneous temperature distribution can occur, viewed in the longitudinal direction of the wire. This is because the location having the highest temperature has the greatest emission and hence is cooled most, so that a self-restoring process occurs. This benefits the stabiliy of the cathode, for example, also in that the temperature of irregularities in the metal wire such as constrictions or inclusions, which would normally assume an extra high temperature and which, consequently, would evaporate particularly rapidly, is thus decreased.

In a preferred embodiment according to the invention a coupling is provided which automatically controls the displacement rate of the metal wire as a function of the temperature of the metal wire in the anode tube. It is favourable not to use the temperature variations of the metal wire as the control signal but rather the resultant variation in the emission of electrons. For

this purpose the non-utilized electron flow incident on the anode tube may be used. In this embodiment the use of a cylinder anode having comparatively small apertures is to be preferred. In other embodiments the anode 2 can also have the form of accelerator anode having a small aperture which is situated near the metal wire.

The invention provides an electron sourcehaving a very small dimension in one direction so that this source is particularly suitable for a scanning electron microscope with scanning in one direction, i.e. in the image direction. The electron beam incident on the object may then have a length which corresponds to the relevant dimension of the object.

The high current density of the electron beam eliminates the drawback of the hardly adequate or inadequate degree of exposure of the object experienced in many applications at a sufficient resolving power. Also in this respect particularly scanning electron microscopes are concerned. So far in certain investigations it was often not possible, due to an inadequate signal, to record relevant radiation or particles such as, for example, a comparatively small spectral range of X-ray or gamma radiation.

An elongated electron source according to the invention is particularly suitable for use in a scanning transmission electron microscope. This is because in known microscopes of this type particularly the drawback of an inadequate signal at permissible, or achievable, current. density in the target plane is experienced. This drawback is eliminated by the said line scanning at such a high current density.

What is claimed is:

1. An electron source comprising a cathode assembly comprising a wire emitter, means for stressing said emitting wire in tension along its longitudinal axis, means for directing an energy beam onto a given portion of said wire to cause electron emmission and means for displacing said wire along said longitudinal axis thereof to vary the point of impingement of the energy beam and increase the emitting life of said cathode assembly.

2. An electron source as claimed in claim 1 wherein said wire essentially consists of a metal. I

3. An electron source as claimed in claim I wherein said displacing means comprises first and second reels, said wire being wound onto said second reel from said first reel.

4. An electron source as claimed in claim 1 wherein said displacing means comprises automatically moving the wire at rates of travel determined by the temperature of the wire. I

5. An electron source as claimed in claim 1 further comprising supporting members for said wire, said supporting members comprising two supports situated on opposite sides of the portion of said wire where the energy beam impinges, said supports being thermal conductors making contact with the wire.

6. An electron source as claimed in claim 1 further comprising a cylindrical anode tube surrounding said portion of said wire where the energy beam impinges, said tube having injection and exit apertures for receiving the energy beam and emitting electrons, respectively.

7. An electron source as claimed in claim 6 wherein said cylindrical anode tube has a length of approximately 5 to mm and an inner diameter of approximately 0.5 to 2 mm.

8. An electron source as claimed in claim 1 wherein said wire essentially consists of tungsten having a thickness substantially between 5 and 50 microns.

9. An electron source as claimed in claim 1 wherein said energy beam is an electron beam and said means for directing an energy beam onto a portion of said wire comprises an auxiliary gun for generating, focussing and directing said electron beam onto said portion of said wire.

It). An electron source as claimed in claim 9 wherein said means for directing an energy beam onto a portion of said wire comprises a minilens for focussing said electron beam 11. An electron source as claimed in claim 1 wherein said means for directing an energy beam comprises an electron source having a high emission current density that results in a homogenizing effect being exerted on local temperature differences within the emissive surface of the wire.

12. A scanning transmission electron microscope comprising an electron source comprising a cathode assembly comprising a metal wire emitter, means for stressing said metal wire in tension along its longitudinal axis, means for directing an energy beam onto a given portion of said metal wire to effect a line emission of electrons from said metal wire, and means for displacing said metal wire along said longitudinal axis, means for forming and directing said electrons into a beam, electro optical devices for controlling said electron beam, means for deflecting said electron beam, means for producing signals representing an object in the path of said electron beam, detecting means for receiving signals from said signal producing means, and a cathode ray tube for producing images of said object from the signals received from the detector, the length of the effective emission portion of the metal wire adapted to cooperate with the electron-optical system so that a resultant line focus corresponds to one dimension of the object measured in the longitudinal direction of the line focus.

13. An electron source comprising a metal wire, means for stressing said metal wire in tension along its longitudinal axis, means for heating said metal wire by an energy beam to cause electron emission, and means for moving said metal wire along in the direction of said longitudinal axis.

14. An electron source as claimed in claim 13 wherein said metal wire essentially consists of tungsten, said tungsten wire being heated by said energy beam means to a temperature of approximately 3,400C and an electron emission current density of at least 50 amperes per cm e l 

1. An electron source comprising a cathode assembly comprising a wire emitter, means for stressing said emitting wire in tension along its longitudinal axis, means for directing an energy beam onto a given portion of said wire to cause electron emmission and means for displacing said wire along said longitudinal axis thereof to vary the point of impingement of the energy beam and increase the emitting life of said cathode assembly.
 2. An electron source as claimed in claim 1 wherein said wire essentially consists of a metal.
 3. An electron source as claimed in claim 1 wherein said displacing means comprises first and second reels, said wire being wound onto said second reel from said first reel.
 4. An electron source as claimed in claim 1 wherein said displacing means comprises automatically moving the wire at rates of travel determined by the temperature of the wire.
 5. An electron source as claimed in claim 1 further comprising supporting members for said wire, said supporting members comprising two supports situated on opposite sides of the portion of said wire where the energy beam impinges, said supports being thermal conductors making contact with the wire.
 6. An electron source as claimed in claim 1 further comprising a cylindrical anode tube surrounding said portion of said wire where the energy beam impinges, said tube having injection and exit apertures for receiving the energy beam and emitting electrons, respectively.
 7. An electron source as claimed in claim 6 wherein said cylindrical anode tube has a length of approximately 5 to 20 mm and an inner diameter of approximately 0.5 to 2 mm.
 8. An electron source as claimed in claim 1 wherein said wire essentially consists of tungsten having a thickness substantially between 5 and 50 microns.
 9. An electron source as claimed in claim 1 wherein said energy beam is an electron beam and said means for directing an energy beam onto a portion of said wire comprises an auxiliary gun for generating, focussing and directing said electron beam onto said portion of said wire.
 10. An electron source as claimed in claim 9 wherein said means for directing an energy beam onto a portion of said wire comprises a minilens for focussing said electron beam.
 11. An electron source as claimed in claim 1 wherein said means for directing an energy beam comprises an electron source having a high emission current density that results in a homogenizing effect being exerted on local temperature differences within the emissive surface of the wire.
 12. A scanning transmission electron microscope comprising an electron source comprising a cathode assembly comprising a metal wire emitteR, means for stressing said metal wire in tension along its longitudinal axis, means for directing an energy beam onto a given portion of said metal wire to effect a line emission of electrons from said metal wire, and means for displacing said metal wire along said longitudinal axis, means for forming and directing said electrons into a beam, electro optical devices for controlling said electron beam, means for deflecting said electron beam, means for producing signals representing an object in the path of said electron beam, detecting means for receiving signals from said signal producing means, and a cathode ray tube for producing images of said object from the signals received from the detector, the length of the effective emission portion of the metal wire adapted to cooperate with the electron-optical system so that a resultant line focus corresponds to one dimension of the object measured in the longitudinal direction of the line focus.
 13. An electron source comprising a metal wire, means for stressing said metal wire in tension along its longitudinal axis, means for heating said metal wire by an energy beam to cause electron emission, and means for moving said metal wire along in the direction of said longitudinal axis.
 14. An electron source as claimed in claim 13 wherein said metal wire essentially consists of tungsten, said tungsten wire being heated by said energy beam means to a temperature of approximately 3,400*C and an electron emission current density of at least 50 amperes per cm2. 